1. Field of the Invention
The present invention relates to a planar microstrip array antenna, and more specifically, to a microstrip array antenna for household use, adapted to receive electromagnetic waves from a broadcast satellite.
2. Description of the Related Art
Conventionally, a parabolic antenna has been used to receive electromagnetic waves transmitted from a broadcast satellite. It is mounted on the roof or balcony of a building so as to be directed to the satellite. The parabolic antenna comprises a reflector, a radiating element, and a converter, the last two being disposed on the focal position of the reflector. Thus, an antenna of this type has a complicated construction, and is large and heavy. In strong winds, such as those of a typhoon, therefore, the parabolic antenna may quite possibly be broken. In snowy areas, moreover, snow may accumulate on the antenna, whereby the electromagnetic waves will be absorbed in it. The installation of the parabolic antenna, furthermore, spoils the external appearance of the building.
Besides the parabolic antenna described above, a planar microstrip array antenna is adapted to receive electromagnetic waves in a frequency band available for broadcast satellites, e.g., a band of about 12 GHz. Since this planar antenna can be mounted along the wall, or the like, of a building, it is less influenced by strong winds, and is less likely to spoil the external appearance of the building.
However, the direction of a beam radiated from the conventional planar microstrip array antenna of this type is perpendicular to the plane direction of the antenna. As shown in FIG. 1, therefore, planar antenna 1 is inclined if it is directed towards broadcast satellite 3. Accordingly, antenna 1 becomes susceptible to strong winds, and snow may accumulate on it, resulting in attenuation of the electromagnetic waves from the broadcast satellite. If the planar antenna is mounted aslant in this manner, moreover, it spoils the external appearance of building 2.
In order to eliminate such an awkward situation, the planar antenna is preferably given a beam tilt or a characteristic such that a beam radiated from the antenna is deviated from a direction perpendicular to the plane of the antenna. In typical latitudes in Japan, planar antenna 1 can be mounted substantially vertically along the wall of building 2, as shown in FIG. 2, by giving the antenna an upward beam tilt of 23.degree., for example. By installing antenna 1 in this way, the influence of strong winds can be reduced, snow can be prevented from accumulating on the antenna, and the effect on the appearance of building 2 can be lessened.
The aforesaid beam tilt can be obtained by giving phase differences to a plurality of radiating elements which constitute an array. FIGS. 3 and 4 show part of the prior art planar microstrip array antenna for circularly polarized waves, constructed as follows. FIG. 3 is a partial plan view of the antenna, and FIG. 4 is a sectional view taken along line 4--4 of FIG. 3. This antenna is formed by superposing first and second printed boards 7 and 8 on earth plate 5, with dielectric layers 6 between them. Feed line 9 with a predetermined pattern is formed on first printed board 7, while a conductor film is deposited on second printed board 8. Part of the conductor film is removed so that a plurality of radiation slots 10 are formed, each with a portion of the conductor film left in the center thereof, thus forming feeding patch 11. Slots 10 and patches 11 constitute a plurality of radiating elements 13a to 13d. Feed line 9 is coupled electromagnetically to feeding patches 11 of the radiating elements. Phase shift portions 12 are formed in the middle of the feed line, whereby a phase delay is caused between each two adjacent radiating elements. This phase delay is adjusted to, e.g., a quarter of wavelength .lambda.g of electromagnetic waves to be propagated. In this arrangement, the beam tilt of about 23.degree. can be given to the antenna.
In order to maximize the antenna efficiency of the planar microstrip array antenna constructed in this manner, the distance between each two adjacent radiating elements must be set to 80 to 90% of wavelength .lambda.o of electromagnetic waves in a free space. In the array antenna with the aforementioned beam tilt, moreover, substantial electromagnetic radiations or grating lobes are inevitably produced in undesired directions. In order to prevent these grating lobes, distance d between the radiating elements in each pair to be given a phase difference must be set to, e.g., 0.64.lambda.o or less. If the array antenna is designed so as to be best suited for the 12-GHz band, the frequency band for broadcasting via satellite, for example, in consideration of these requirements, the outside diameter of radiating slot 10 of each radiating element is about 14 mm, and distance d is about 16 mm. Accordingly, the gap between the outer peripheral edges of the respective radiating slots of each pair of radiating elements to be given the phase difference is about 2 mm, which is not a very wide space. Since phase shift portions 12 are formed in the middle of the terminal portions of feed line 9, moreover, the configuration of the feed line is complicated. At such portions as those indicated by symbols A, B and C in FIG. 3, therefore, the feed line is situated so close to the radiating elements that undesired electromagnetic coupling are caused between them, thus lowering the gain of the antenna. If the width of the feed line is reduced to enlarge the distance between the feed line and the radiating elements, in order to prevent these undesired electromagnetic coupling, a great loss is produced in the feed line, so that the antenna gain is lowered.
As described above, the conventional planar array antenna with a beam tilt entails reduced gain. If the configuration of the feed line is thus complicated, moreover, the phase is asymmetrical at the diverging and bent portions. Accordingly, impedance matching is difficult, and again, the gain is lowered.